The present invention relates to an internal combustion engine and a method for operating it.
To open an inwardly opening, high-pressure fuel injection solenoid valve used with gasoline direct injection, the high system pressure makes it necessary to include a booster phase, in which the current flowing through the high pressure injector increases to values such as 12 A. The high current is generated by connecting the high pressure injector to a booster capacitor that stores energy with a voltage of, e.g., 65 V, and delivers it to the high pressure injector during the booster phase. The energy withdrawn in the booster phase is resupplied to the booster capacitor by a reload circuit before the next booster phase. The size of this reload circuit and the booster capacitor depends, among other things, on the booster energy required by the high pressure injector which, in turn, depends on the booster current required to open the high pressure injector. The level of the booster current is determined primarily by the maximum system pressure against which the high pressure injector must open, and by the static flow rate.
The highest system pressure that exists during normal operation in systems with gasoline direct injection is determined by opening a pressure-limiting valve. The opening pressure of the pressure-limiting value is attained in two cases of normal operation. The first case is a hot start, i.e., a starting procedure after a shutoff phase, which is accompanied by an increase in pressure in the high pressure fuel system due to the fuel heating up. The fuel in the fuel system is heated up the by the heat transferred from an engine that had been previously driven under full load and was therefore heated up to an extreme extent. The second case is the resumption of fuel injection after an overrun condition. In an overrun condition, fuel injection is halted, and pressure increases in the high pressure fuel system for the reason given above. In both cases, the pressure in the high pressure fuel system is lowered after a few injections to a normal, lower pressure level. The booster current is designed in accordance with the maximum attainable pressure, however, which is the opening pressure of the pressure-limiting valve. This means that the reload circuit and the booster capacitor are oversized for normal operation.
The object of the present invention, therefore, is to provide a method that ensures reliable injection, even in extreme cases, such as resumption of fuel injection after an overrun condition, and in a starting procedure after a shutoff phase that is accompanied by an increase in pressure in the high pressure fuel system due to the fuel heating up, when booster capacitors are used that were designed for normal operation.